The goal of this project is to provide support of National Toxicology Program (NTP) hazard identification activities targeted toward the prevention of diseases or adverse effects caused by environmental exposure to chemical or physical agents. Toxicity testing is an important aspect of public health research in that it serves to identify chemicals that are hazardous to human health. Proper conduct of toxicology studies is required to ensure that the resulting data are reliable. This project involves characterization of the toxicity of C9-alkylbenzenes, including 1,2,4-trimethylbenzene, 2-ethyltoluene, and mixed xylenes, following exposure via whole body inhalation. Humans are likely to be similarly exposed to these compounds via inhalation. The data from this project will be used in the development of sound, scientific conclusions about the potential toxicity and carcinogenicity of 1,2,4-trimethylbenzene and 2-ethyltoluene in rats and mice and ultimately supports the risk assessment efforts of National Toxicology Program and other federal agencies. In addition, the public can be adequately informed about risk factors arising from exposure to 1,2,4-trimethylbenzene and 2-ethyltoluene. Keywords: toxicity, inhalation, trimethylbenzene, ethyltoluene, xylene, lung